The present invention relates generally to Code Division Multiple Access (CDMA) systems, and more particularly, to radio resource control signaling in CDMA systems.
High Speed Downlink Packet Access (HSDPA) is packet data service offered in Wideband Code Division Multiple Access (WCDMA) networks. The HSDPA is an evolution of WCDMA specified by the Third Generation Partnership Project (3GPP) in Release 99 of the WCDMA standard. The HSDPA, introduced in Release 5 of the WCDMA standard, provides peak data rates up to 10 Mbits/s using enhanced features such as higher-order modulation (16 QAM), physical layer hybrid automatic repeat request (H-ARQ), multicode transmission, fast link adaptation, and fast scheduling. The transport channel for HSDPA is the High Speed Downlink Shared Channel (HS-DSCH). The HS-DSCH is carried over the High Speed Physical Downlink Shared Channel (HS-PDSCH).
The HS-DSCH is a time multiplexed channel shared by a plurality of mobile stations. Mobile stations are scheduled to receive data transmitted on the HS-PDSCH by a serving base station. The scheduling interval is referred to as a Transmission Time Interval (TTI). During a given TTI, one or more mobile stations may be scheduled. The mobile stations report channel conditions to the base station on an uplink channel called the High Speed Dedicated Physical Control Channel (HS-DPCCH) to enable the base station to make scheduling decisions. The base station schedules the mobile station based, at least in part, on the reported channel conditions. The identity of the mobile stations scheduled to receive packet data on the HS-DSCH in a given TTI is transmitted on the High Speed Shared Control Channel (HS-SCCH). The HS-SCCH is also used to send transmission parameters needed by the mobile station to decode the HS-DSCH, such as the code channels, the transport block size, and the modulation scheme used in the corresponding TTI.
Unlike Dedicated Physical Channels (DPCH) in WCDMA, soft handoff is not supported for HSDPA. Due to the complexity of coordinating packet data transmissions between cells, hard handover (HHO) is used. The mobile station measures the instantaneous signal-to-intererence ratio of a pilot signal received from each cell in its active set and requests service from the cell providing the strongest signal. As the mobile station moves into a boundary zone between cells, the signal strength from the serving cell will diminish while the signal strength from a neighboring cell in its active set will increase. When the signal strength from the neighboring cell exceeds the signal strength from the current serving cell, the mobile station requests a handover from the current serving cell to a specified target cell. When the current serving base station acknowledges the handover request, the mobile station switches to the target cell and sends a handover complete message to the target base station to complete the handover. The target base station assumes the role as the serving base station and begins transmitting packet data to the mobile station.
The HS-DSCH, according to release 5 of the WCDMA standard, always operates in conjunction with an Associated Dedicated Physical Channel (A-DPCH) in both the uplink and downlink. The A-DPCH carries Radio Resource Control (RRC) messages between the mobile station and the base station. Radio resource control is a protocol that provides control of the mobile station by a radio network controller in a radio access network. The RRC functions include handover control of the mobile station in connected mode. Release 6 of the WCDMA standard allows a serving base station to send RRC signaling messages, including handover messages, to the mobile station in band over the HS-PDSCH instead of the A-DPCH.
The handover procedure typically takes about 200-800 ms to complete from the time that the mobile station sends the handover request. During that period, the signal quality on the HS-PDSCH from the serving cell may vary significantly. If the HS-PDSCH is used for sending handover messages to the mobile station instead of the A-DSCH, there is a greater risk of packet loss. If the channel conditions deteriorate too much, the mobile station may not be able to receive handover message from the serving base station, which may prevent the mobile station from completing the handover and may result in radio link failure, i.e. a dropped call.
Therefore, there is a need to make signaling between the base station and the mobile station more robust, particularly when in band signaling on the HS-DSCH is used to transmit handover messages to the mobile station.